You maintain something when you keep it in its current state. For example, you might maintain a friendship with someone, or maintain your house to keep it in good condition. This is called carrying out maintenance.
The soldiers in the picture are maintaining grave sites.
Birds or animals migrate when they all move to a new area at the same time. Migrations happen with the seasons at the same time each year so the birds or animals follow their food source. Sometimes they travel an incredible distance - some kinds of bird travel halfway around the world!
Mixing is a way of combining two or more things together so you cannot easily tell what went into the mix.
You make cake batter by mixing the ingredients together, and you make new colours of paint by mixing different colours together. Food described as 'mixed' like mixed candy or mixed nuts is made from several different kinds of food mixed together.
The picture is of someone mixing several different colours of paint together.
Moulting, also spelt molting, is when a bird or animal loses some of its feathers or fur so that it can grow new ones. Birds and animals usually moult on the change of season, so that they can grow a thick coat for the winter, or shed their winter coat when the weather warms up.